1. Field of the Invention
The present invention relates to a composite for an anode material, and anode materials and a lithium battery using the composite for an anode material, and more specifically, to a composite for an anode material having a large charge and discharge capacity and good capacity retention, and an anode material and a lithium battery having the composite for an anode material.
2. Description of the Related Art
Lithium has been used as an anode material for conventional lithium batteries. However, if lithium is used as the anode material, a short circuit of a battery occurs due to the formation of dendrite, and thus, there is a high possibility of explosion. Accordingly, a carbon group material has been used as the anode material instead of lithium.
The carbon group anode material includes crystalline carbon such as graphite and artificial graphite and amorphous carbon such as soft carbon and hard carbon. Amorphous carbons have a large capacity. However, they have a high irreversibility during charge and discharge process. Graphite is a representative crystalline carbon, and is used as an anode material due to its high capacity, that is, graphite has a theoretical capacity limit of 372 mAh/g. However, although such graphite and carbon group materials have a high theoretical capacity limit, the theoretical capacity limit remains only around 380 mAh/g. Thus, such graphite and carbon group materials can hardly be used as a anode in high capacity lithium batteries.
In order to overcome the above problem, presently, anode materials belonging to a metal group or an inter-metallic compound group have been actively studied. For example, lithium batteries that use metals such as Al, Ge, Si, Sn, Zn, or Pb or a semi-metal as an anode material have been studied. These materials have high capacity and high energy density, and can occlude and emit an amount of lithium ions larger than an anode material that uses a carbon group material, and thus, it is believed that a battery having a high capacity and a high energy density can be manufactured using the above mentioned materials. For example, it is known that pure silicon has a theoretical capacity of as high as 4017 mAh/g.
However, these materials have a reduced cycle life compared to the carbon group materials, and thus, it is yet difficult to put them to practical use. If inorganic particles of Si or Sn are used as the anode material, that is, a material to insert or extract lithium, the electrical conductivity between the active materials is reduced or an anode material is exfoliated from a current collector due to change in volume during charging and discharging processes. That is, the inorganic particles such as Si or Sn included in the anode material expand their volume by 300 to 400% by inserting lithium during charging. When lithium is extracted from the inorganic particles by discharging, the volume of the inorganic particles is contracted. In this way, when the charge and discharge cycles are repeated, cavities are formed between the inorganic particles and the anode material, and thus, an electrical insulation can be generated, which results in a rapid reduction in cycle life of the anode material. For this reason, the practical use of these materials is still hindered in a lithium battery.
In order to overcome the problems described above, an anode material that a mixture of flakes of metal or alloy powder, flakes of carbon powder, and binder, capable of reversibly storing/discharging lithium has been disclosed in Japanese Patent Publication No. 1994-318454. However, since the anode material is merely a composite anode material that has simply been mixed, stresses occur due to expansion and contraction of a metal according to the repetition of charge and discharge processes, and there is a severe disconnection of electron moving path.
According to J. Yang et al., (Solid state ionics, vol. 152-153, p 125), when SiOx (x=0.8, 1.0, and 1.1) is used as an anode material, as the oxygen content is reduced, initial efficiency increases, however, capacity retention per cycle decreases.
According to Japanese Patent Publication No. 2000-243396, a carbon-SiO composite made by compositing SiO and graphite or a carbon precursor (pitch) is used as an anode material. However, it cannot be called a high capacity anode material since it substantially has a charge and discharge capacity of approximately 350 mAh/g.
In Korean Patent Application No. 2004-7016728, composite particles of Si, SiOx, and carbon particles, and particles of these materials, which are coated with a carbon material, are used as anode materials. However, it can be said that the anode materials have not used the characteristics of SiOx to their full potentials compared to the present invention since the anode material is a mixture of SiOx particles.
Therefore, there is a need to develop an anode material having a large capacity and an improved capacity retention, and a lithium battery having an improved cycle efficiency by using the anode material.